6.1 Conditional programming with if statements

The conditional if statement is used to test an expression. If the test_expression is TRUE, the statement gets executed. But if it’s FALSE, nothing happens.

# syntax of if statement
if (test_expression) {
        statement
}

The following is an example that tests if any values in a vector are negative. Notice there are two ways to write this if statement; since the body of the statement is only one line you can write it with or without curly braces. I recommend getting in the habit of using curly braces, that way if you build onto if statements with additional functions in the body or add an else statement later you will not run into issues with unexpected code procedures.

x <- c(8, 3, -2, 5)

# without curly braces
if(any(x < 0)) print("x contains negative numbers")
[1] "x contains negative numbers"

# with curly braces produces same result
if(any(x < 0)){
        print("x contains negative numbers")
}
[1] "x contains negative numbers"

# an if statement in which the test expression is FALSE
# does not produce any output
y <- c(8, 3, 2, 5)

if(any(y < 0)){
        print("y contains negative numbers")
}